1. Field of the Invention
The present invention relates to a code division multiplex transmitting and receiving apparatus and a method of carrying out code division multiplexed communication between a central station and a plurality of receiving apparatuses.
2. Description of the Related Art
The following documents are referred to below.    Patent document 1: Matsuno et al., Japanese Patent No. 3913139, CDMA transmitter, CDMA multiplex transmitter, CDMA receiver, and CDMA communication system, pre-grant publication Mar. 11, 2004 as JP 2004-080385    Patent document 2: Sasaki et al., JP 2003-317026, Signed product sum computing element and analog matched filter including the same, published Nov. 7, 2003    Non-patent document 1: Tamai et al., ‘Jisedai hikari akusesu shisutemu COF-PON no kenkyu kaihatsu’ (Research and development of COF-PON: a next-generation optical access system), Oki Technical Review, Issue 210, Vol. 74, No. 2, April 2007    Non-patent document 2: Kashima et al., ‘Ko-QoS maruchi media hikari haishin shisutemu no kenkyu kaihatsu—COF transhiba’ (Research and development of high-QoS multimedia optical distribution system—COF transceiver) Oki Technical Review, Issue 200, Vol. 71, No. 4, October 2004    Non-patent document 3: Sasase, ‘Hikari shisutemu ni okeru hikari fugo bunkatsu tagen setsuzoku gijutsu’ (Optical Code Division Multiple Access Techniques in Optical Communication Systems) TELECOMFRONTIER, November 2004
Code division multiplexing (CDM) is currently employed in mobile access network systems, where it provides the capability to carry high volumes of communication traffic on multiple channels while conserving frequency and time-slot resources. Synchronous CDM systems in which each channel is separately synchronized have the particular advantage of providing stable extraction of an arbitrary channel from the multiplexed signal, as described by Matsuno et al. in the above patent document 1.
Passive optical network (PON) systems that use CDM for fiber-optic communication between a provider and stationary users are also attracting attention. Known as CDM-on-fiber-PON or COF-PON systems, these systems permit transmission over longer distances than are feasible in more traditional time division multiplex (TDM) optical access systems. COF-PON also has the advantage of being compatible with wavelength division multiplexing (WDM). COF-PON systems are described in non-patent documents 1-3 and patent document 2.
CDM transmitting and receiving apparatus is thus needed both for COF-PON and other optical access network systems, and for mobile communications. The present apparatus is concerned with apparatus for transmitting and receiving a CDM signal that is sent from the central station to a plurality of terminal units. The transmitting apparatus is often referred to as central office apparatus or optical line termination apparatus in optical access networks and as a base station apparatus in mobile communication systems. The terminal units may be referred to as optical network units in optical access systems, as mobile stations in mobile communication systems, and as subscriber apparatus in both types of systems.
In conventional CDM communication systems, typified by the system described by Matsuno et al. in patent document 1, the transmission rate per channel is fixed, and the maximum distance from the transmitting equipment to the receiving equipment is also fixed. These system parameters are related to the coding rate, that is, the number of chips into which each data bit is divided when transmitted, and the chip rate, that is, the number of chips transmitted per second. Modifying these parameters to accommodate a subscriber requiring a particularly high data rate or a subscriber located particularly far from the transmitting apparatus is not easy.
Accordingly, a conventional system must be designed to accommodate the needs of the most distant anticipated subscriber, and the subscriber with the highest anticipated transmission rate, even though most subscribers may have less demanding requirements. As a result, much of the capacity of the system becomes excess capacity that is not used. Furthermore, if an unanticipated new subscriber joins the system and the subscriber's communication needs exceed the system capabilities in terms of distance or transmission rate, the entire central office apparatus must be modified or replaced.
It would be preferable for the transmitting and receiving apparatus to more flexible, so that a long-distance or high-data-rate transmission capability could be provided for particular subscribers without being provided for all subscribers. A flexible system of this type would be far less expensive to operate than a conventional fixed-parameter system.
Diligent study of this problem by the present inventor has shown that a feasible solution is to generate two parallel data signals per channel, code them in parallel with different codes, convert the two coded signals to a single serial signal, and multiplex the serial signal.